1. Field of the Invention
The invention relates generally to methods of refurbishing or restoring metal components back to an acceptable operational condition using subtractive surface engineering techniques that maintain the component within geometrical tolerance. The method is particularly applicable to components manufactured or finished to tight tolerances that are used in metal to metal contact mechanisms and where the original manufacturing geometric specification may be absent or unavailable. The method further relates to a method of assessment of such components for refurbishment and the refurbished products thereof.
2. Description of the Related Art
Used, worn or damaged high value metal components and new components damaged during storage, handling, assembly or transportation, including cam shafts, crank shafts, bearings, gears and the like, can sometimes be refurbished by regrinding or re-machining (e.g. milling, lathing and the like) the component's critical used surfaces. If the operation is successful, the component may be put back into service at less cost than would have been the case were the component replaced by a new part. In order to do this, however, the machinist must have a copy of the component's Engineering Specification Drawing (ESD) or equivalent specification sheet to be able to correctly refurbish the critical surfaces. The ESD will contain information such as all dimensions used to originally manufacturer the component, the tolerances on all dimensions, the component's material and heat treatment, and the like. This information is needed to allow the machinist to correctly regrind or re-machine the component's critical surfaces and to inspect the results.
Also, often complex and expensive Component Specific Tooling (CST) is required to fixture the metal component for any regrinding or re-machining operation and/or component specific inspections. The machinist must have a set of this CST, or be able to manufacture suitable tooling to fixture and/or inspect the component.
Since the refurbishment is often done at a facility other than that of the Original Equipment Manufacturer (OEM), the ESD and/or CST are likely to be unavailable and probably unattainable from the OEM. In fact many OEMs do not make their ESDs available to third parties. In all likelihood then, these components would be scrapped at great expense. In many cases, replacement components are no longer manufactured or require a long lead time to purchase. This can lead to costly lost machine availability or to the premature retirement of the entire machine from which the used component came.
In addition, even if the ESD and CST are available, a considerable amount of manpower and expensive equipment is needed in setting up and carrying out the regrinding or re-machining process. For just one individual item, the cost of re-machining may not justify the effort required. This is often the case if a single machine is overhauled; a small number of different components with varying shapes and sizes will need to be refurbished. The cost of refurbishment by a regrinding or re-machining process may very well be too expensive to be commercially viable.
An additional problem is that of retaining the original tolerances. In certain circumstances, regrinding may remove so much material that the component becomes undersized. This cannot always be determined prior to commencing work and the high levels of scrap in such processes considerably increase the overall cost of the work. Usually a regrinding operation will comprise setting up and aligning the component in the grinder or lathe, performing a first pass, inspecting and adjusting the alignment of the component and performing a further pass to remove the desired quantity of material. Sometimes, a number of passes may be required merely to achieve correct alignment. In certain processes, the minimum amount of material that can be effectively ground in a single pass is 10-20 microns. If three passes are required to complete the component, as much as 60 microns may have been removed. For e.g. a gear tooth in which material has been removed from both faces of the tooth, a total dimensional change of 120 microns may result.
An additional problem is that these refurbishing methods can result in surface material movement, deformation, impregnation, tearing, smearing and/or metal overlapping. These forms of material distress hereinafter referred to as “surface distortion” can mask the effectiveness of inspection techniques such that the surface damage cannot be identified and the component could be put back into service without having been successfully restored.
Superfinishing of engineering components at a final stage of production has been known for a number of years. One method of superfinishing is a chemically accelerated vibratory finishing procedure available from REM Chemicals, Inc. The procedure uses an active chemistry such as a mildly acidic phosphate solution which is introduced with the component into a vibratory finishing apparatus together with a quantity of non-abrasive media. The chemistry is capable of forming a relatively soft conversion coating on the metal surface of the component. Vibratory action of the media elements will only remove the coating from asperity peaks, leaving depressed areas of the coating intact. By constantly wetting the metal surface with the active chemistry, the coating will continuously re-form, covering those areas where the bare underlying metal has been freshly exposed, to provide a new layer. If that portion remains higher than the adjacent areas it will continue to be rubbed away until any roughness has been virtually eliminated. A general description of this superfinishing process is provided in commonly owned U.S. Pat. Nos. 4,491,500 4,818,333 and 7,005,080 and U.S. Patent Publication Nos. US 2002-0106978 and US 2002-0088773 each of which is incorporated herein by reference. Application of such a process to surfaces of large sized gears is described in WO2004/108356, the contents of which are also incorporated herein by reference.
Studies have been performed to determine the utility of such processes in the refurbishment of used gears. Based on such studies it has been determined that a beneficial effect may indeed be achieved in removing damage such as foreign object damage (FOD), scoring, micropitting, pitting, spalling, corrosion, and the like. The extent to which components could be refurbished was hitherto determined by the depth of the damage according to an initial inspection of the parts. For gears where the depth of the damage was less than 0.1× the AGMA (American Gear Manufacturers Association) recommended maximum backlash, refurbishment was generally considered possible. For damage exceeding this depth, the part was generally recommended for scrap. Based on this damage assessment, a large proportion of the gears initially assessed were not deemed suitable for refurbishment. Additionally, of those components where refurbishment using superfinishing was carried out, a number of the components were subsequently scrapped after treatment due to the presence of excessive damage that only became apparent on treatment. In these cases, not only was the component scrapped but the time taken to perform a complete refurbishment cycle was also wasted.
Procedures are available for non-destructive testing of metallic components to determine the extent of surface damage. Such procedures including photomicrography and fluorescent penetrant inspection are however highly complex and their performance adds greatly to the overall cost of a refurbishment procedure. It would thus be desirable to have an improved procedure for assessing candidate components for refurbishment that allows more components to be recovered without unnecessarily adding to the overall cost and time per successfully recovered component.